The current paper seeks to discuss the process of sedimentation through which different types of sedimentary rock formation occur. The weathering of materials produces sediments that are transported, and deposition puts these sediments at the place at which new rocks form. This process is followed by diagenesis through which the deposits undergo changes due to the actions of living organisms, chemical replacements, as well as solution and re-deposition of permeating water. Consolidation compacts hard rock masses using loose sediment. Clastic, chemical, and organic are the main three types of rocks formed through the process of sedimentation. The clastic rocks consist of the rudites, lutites, and arenites, which are classified according to the sizes of sediment particles causing their formation. Chemical rocks are formed through either the precipitation of ions in aqueous solutions or evaporation of water from ion solutions leaving the evaporated substances such as salt behind. Crystalline limestone and coal are some of the primary examples of chemical and organic sedimentary rocks. Sedimentary rocks are important because they yield valuable historical information about the earth, are used in construction, and represent a valuable energy resource.
Keywords: Sedimentary rocks, sedimentation
Sedimentation and Sedimentary Rocks
Rocks are important resources located on the surface of the earth because they provide raw materials for construction, host water reserves that benefit the living organisms, provide fuel, and other minerals. Sedimentary rocks form on the surface of the earth under normal pressure and temperature after the accumulation of weathering products of both organic and inorganic materials (Thorpe & Thorpe, 2009). Weathering refers to the process of chemical and physical breakdown of rocks and other materials on the surface of the earth by means of such factors as abrasion, rain, and wind. The process of weathering results in the formation of sediments, which represent loose grains and chemical residues of the weathering materials that include parts of plants and animals, rust, mineral grains, and rock fragments. Sedimentation results in the formation of many types of sedimentary rocks, which are valuable not only to humans but also to other living organisms.
Formation of Sedimentary Rocks (Sedimentation Process)
The process of sedimentary rock formation involves several steps starting from the process of weathering. During weathering, rocks, minerals, and organic materials on the surface of the earth are broken down to form small particles called sediments (Thorpe & Thorpe, 2009). The sediments are classified according to their sizes. They include clay, silt, sand, pebble, cobble, and boulder. Acids, ice, water, temperature variations, plants, and animals are among some of the leading agents of weathering (Nelson, 2015). Chemical and mechanical weathering represents two major types of weathering. In mechanical weathering water from rain and other sources accumulates in rock cracks and with temperature variations, the water may turn into ice, which increases its volume in cracks breaking the rock into particles. Furthermore, the process occurs due to the impact of animals when walking or digging into the rocks as well as when plant roots grow into cracks breaking the rock. Furthermore, the temperature change that compels rocks to heat up and cool causes physical weathering. Chemical weathering occurs under the influence of chemical solutions such as sulfuric and carbonic acids, which dissolve rocks.The particles or sediments formed during this process are then transported from the site of weathering.
Erosion, Transportation, and Deposition
After weathering, erosion and transpiration of sediments take place through which particles move from the site of weathering for deposition to places where the rocks form. Rivers, seas and other water bodies, glaciers, winds, and landslides are the main agents of sediment erosion and transportation (Nelson, 2015). Erosion is the initial stage of sediment transportation from the area of weathering to the location of deposition. Weathered debris move under the influence of ice, water, and wind, or through gravity by mass wasting and rock falls. Transportation of the sediments can further occur by moving down slope, being carried by the wind, water in the rivers and streams, and ocean currents. After erosion and transportation, sediment deposition follows once the energy of the medium of transport accumulates to move the weathered particles. According to Thorpe and Thorpe (2009), the place of deposition reflects the energy of the medium of transport. However, some sedimentary rocks like evaporates are composed of materials formed at the site of the deposition, meaning that some rocks may form without going through the process of deposition. Since sediment deposition occurs at different times, stratification or bedding is a common feature of sedimentary rocks. The rocks form in layers or strata whose thickness vary depending on how deposition occurred.
Diagenesis and Lithification
Diagenesis and lithification are the final stages of the sedimentation process through which sedimentary rocks are formed. The process of diagenesis includes various kinds of changes occurring in the deposited sediments from the time of deposition to the time when complete consolidation occurs (Thorpe & Thorpe, 2009). The changes emanate from digestive processes of living organisms, bacterial activity, chemical replacement, and solution and re-deposition of permeating water. On the other hand, lithification or consolidation is the process through which deposited and weathered loose unconsolidated sediments transform into a cohesive sedimentary rock.
Chemical, physical, and biological processes mark the term diagenesis. They include compaction, deformation, dissolution, cementation, replacement, recrystallization, hydration, bacterial activity, and development of concretions (Mibei, 2014). Rocks that are formed at the point of weathering with the transportation of sediments are called sedentary sedimentary rocks. As a rule, sediments such as sand and pebbles are deposited as loose masses that form unconsolidated rocks. Diagenesis occurs under the conditions of low temperature and pressure because the deposited sediments are close to the surface of the earth.
During this process of diagenesis and depending on the type of sediments, reactions occur between the connate water trapped in the rocks during deposition and the minerals in the rock, as well as the percolating groundwater. Organic activity may also lead to the formation of weak acids, which dissolve carbonates and other compounds, and may possibly result in re-depositing them to other places (Nelson, 2015). Bacterial organisms also act on sulfates reducing them to sulfides, which further release acids that react with carbonates. That notwithstanding, animals such as worms can burrow through the deposited sediments destroying their original forms through such actions as ingestion of the soil to form new soil textures among others.
Consolidation, which occurs during the process called lithification, is the final process in which chemical and physical reactions convert the sediments into a consolidated rock from the unconsolidated form. Three main processes are involved in their formation, comprising of compaction, re-crystallization, and cementation (Thorpe, 2012). During compaction, the weight of the deposits compresses the sediments into a mass. In the process of deposition, the sediments may be arranged in layers with the ones at the bottom being under considerable pressure. As pressure piles up because of the weight of the materials that continue to settle in the area of deposition, much of the connate water is expelled forcing the sediments to come together. As the grains are compelled by the pressure of the overlying layers, their outer surfaces may dissolve and re-crystalize, resulting in the welding of the grains together.
In the process of re-crystallization, the pressure is still immense in the lower layers of the sediments. Percolating water dissolves some materials from one area before re-depositing them elsewhere (Thorpe, 2012). Alternatively, the water can make new substances penetrate the deposited sediments, hence resulting in re-crystallization. After re-crystallization, cementation occurs whereby the deposits from aqueous solutions can be found in voids or other spaces between the sediments. The crystallization of these solutions binds the sediments together, and through this process, the loose rock aggregates are converted into a solid, coherent sedimentary rock.
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Classification of Sedimentary Rocks
There are three types of sedimentary rocks. They include clastic rocks made of grains that are produced from mechanical weathering products and chemical rocks formed mainly through chemical processes resulting in the precipitation of compounds from aqueous solutions (Thorpe, 2012). Organic sedimentary rocks are formed from organic debris like plant and animal debris.
Clastic rocks are comprised of three main classes of rocks. They include rudites, arenites, and lutites made from weathered grains of more than 2mm, 2-0.05mm, and less the 0.05mm respectively (Khullar, 2012). Conglomerate and breccia are rudites since they contain large coarse-grained blasts such as boulders, cobbles, and pebbles. In case of conglomerate rocks, the coarse-grained clasts are rounded, something that indicates that they have spent considerable amount of time in the process of erosion and transportation before being deposited in the high energy environment (Nelson, 2015). The breccia is formed from coarse-grained clasts that are angular, which shows that these sediments spend a short period of time in the transportation cycle compared to the sediments that form conglomerate.
The next group of clastic rocks comprises of the arenites, made from smaller particles than the rudites. Sandstone, which is the main rock in this group, is made of sand-sized particles (Khullar, 2012). Sandstone texture and composition allow the historical interpretation of both the depositional and transportation cycles along with the determination of their source. Quartz is a dominant mineral in sandstones and rocks that are called quartzite. An arkose arenite rock contains abundant feldspar while a lithic sandstone is made of small rock fragments. Lutites comprise of the mudstones and shale. Mudstones are made of fine clay-sized particles usually invisible to the naked eye and consist of clay and quartz minerals (Khullar, 2012; Thorpe & Thorpe, 2009). Just like mudstones, shale is composed of clay-sized particles with a difference that it breaks into thin flat fragments. A mudstone does not break into these kinds of fragments. The lutites are formed in deep ocean lakes and ocean basins because the deposition of the small microscopic particles occurs in non-agitated calm water.
Organic Sedimentary Rocks
Just like the name suggests, these rocks are made from organic sediments or debris from plants and animals. Biochemical Limestone is an example of these organic rocks that forms when calcite precipitates. In biochemical cherts, tiny silica-secreting organisms resembling planktons (diatoms) accumulate on the seabed, recrystallizing to form the biochemical cherts (Thorpe & Thorpe, 2009). If these diatoms fail to recrystallize, diatomite is formed. Coal is also an organic rock that forms after the accumulation of plant fossils, meaning that its main component is carbon. The process of the formation of this rock requires anaerobic conditions so that if the organic matter is buried and with greater pressure, heat, and time, the deposits dehydrate transforming from the spongy peat to soft and hard coal (Thorpe, 2012). Lignite is a soft brown coal containing minimal amounts of carbon and energy compared to the hard anthracite and bituminous forms of coal.
Chemical Sedimentary Rocks
During the process of weathering and transportation, dissolved ions find their way into streams and ground water ending up in oceans and seas. The concentration of these ions may become high resulting in the precipitation forming a mineral that constitutes rocks called precipitates. Crystalline limestone and travertine are an example of precipitates that contain calcium carbonate (Khullar, 2012). Recrystallization of this limestone by the action of magnesium rich fluids forms dolomite rock. Others include the iron-rich oolites and silica-rich rocks such as flint. On the other hand, the water may evaporate producing rocks called evaporates such as salt and gypsum among many others.
Importance of Sedimentary Rocks
These rocks contain essential information concerning the history of the earth. Through the use of fossils that preserve the remains of both ancient plants and animals, scientists can understand environmental changes that have taken place over time. This is because sedimentation under high pressure and temperature does not destroy the fossil fuels (Mibei, 2014). Furthermore, these rocks are soft and easy to cut, something that enables them to be used in the construction industry. Limestone is also a raw material for cement. Most importantly, some of these rocks are a source of energy, especially coal. Additionally, sedimentary basins represent important reservoirs for hydrocarbons and petroleum, which are critical resources for the energy industry.
Sedimentation process leads to the formation of different types of sedimentary rocks. This process begins with weathering to create small sediments that are then transported and deposited in the places of rock formation under the influence of various transport agents. Deposition occurs at different times, something that gives some of these rocks a stratified look. Diagenesis and lithification occur before the actual rock is formed. They are classified as clastic, organic, and chemical rocks. Conglomerate, breccia, sandstones, and mudstones are all clastic rocks while crystalline limestone and coal are the examples of chemical and organic rocks respectively. Sedimentary rocks are useful because they store valuable historical information about the earth. They are also useful for construction and energy industries.